Good Fruit Grower

March 15

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The apple industry uses
Manchurian crab as a pollinizer
because of its compatibility with
the major apple varieties. This
Manchurian is pictured in a
Gala orchard.
It also needs to bloom annually and
have good pollen viability and germination. Research at the Virginia Polytechnic
Institute and State University, suggests
that the pollen of Manchurian has a relatively slow pollen tube growth rate and so
might not be the most efficient pollinizer
(see "Optimizing pollination").
Now, it appears that disease resistance
is important, too.
"I think it's time to get rid of
Manchurian," said Auvil, who has planted
Manchurian in every block in his own
orchard since 1990.
The ornamental wholesale nursery J.
Frank Schmidt & Son, of Boring, Oregon,
lists 39 crab apple varieties on its Web site,
but little is known about their suitability
as pollinizers in commercial orchards.
Dolgo, Jackii, and Pink Spires are the
earliest bloomers.
Dolgo is thought to be a good candidate as a pollinizer as it has an upright
growth habit, white blossoms, and good
disease resistance. It is also one of the
hardiest crabs.
Jackii produces profuse and long-lasting white blossoms and is hardy. It is
resistant to scab, cedar-apple rust,
mildew, and has some resistance to fireblight. The tree, however, is rounder
(growing 20 feet tall and 20 feet wide) and
not as tall as Dolgo.
Pink Spires would not fit the apple
industry's needs because of its bright pink
flowers and reddish purple foliage.
Sugar Tyme blooms a little later but has
potential because of its white flowers and
upright growth habit.
photos courtesy of willow drive nursery
Trial
www.goodfruit.com
Mike Willett, vice president for scientific affairs at the Northwest Horticultural
Council, said good crab apple candidates
need to be identified and then evaluated
alongside the standard Manchurian crab.
The most efficient way to do this would be
in a research trial, rather than by growers
individually. The question is, who would
be able to organize such a trial?
Auvil said promising cultivars need to
be screened for compatibility and performance with major fresh varieties that
the industry grows. Characteristics that
need to be evaluated include attractiveness to bees, tree structure, and disease
tolerance.
Washington State University recently
appointed horticulturist Dr. Desmond
Layne to a new technology transfer position with the tree fruit industry, and Dr.
Stefano Musacchi, a horticulturist from
Italy, will join WSU in August. Auvil said
it's possible that one of the new faculty
might propose to lead such a project.
•
Optimizing pollination
Scientists hope to find out how the pollen
source affects pollination and final fruit
size and quality.
by Geraldine Warner
R
esearchers at Virginia Tech are studying pollen tube growth in apple pollinizers
in an effort to help growers optimize pollination.
The fertilization process begins after pollen is deposited on a flower stigma.
Male gametes from the pollen are transported through a pollen tube that
grows from the pollen grain down through the style to the ovules where the
egg is fertilized, resulting in fruit set ("An apple flower").
Dr. Keith Yoder, horticulturist at Virginia
Tech, said understanding the progression
of pollen tube growth is critical in order to
apply bloom thinners at the right time.
Style
Stigma
Yoder and his colleagues developed a temperature-based model that calculates the
Anther
time required to fertilize the king bloom
after pollination. The model has been beta
tested in Washington for the past two years
via the AgWeatherNet Web site, where it is
integrated with current and forecasted
weather data.
The idea behind the model is to help
growers predict when a large enough percentage of the flowers has been fertilized
Filament
Nectarto set a good crop of large fruit. Then, they
secretory
Sepal
can apply bloom thinners, such as liquid
glands
lime sulfur and fish oil, before more pollen
Ovary
tubes reach the flower ovules and set more
fruit. The goal is to help them save money
Ovule
by applying sprays at the optimal timing
Embryo
and avoiding unnecessary thinning
sac
sprays.
"It's a way to add precision to the way
we're using bloom thinners, instead of saying apply them at 20 or 80 percent bloom," Dr.
Gregory Peck, horticulturist at Virginia Tech, reported to the Washington Tree Fruit
Research Commission, which has helped fund the work.
While the pollen tube growth rate is built into the model, the
grower must measure the average style length to determine the
distance the pollen tube has to grow, and enter this into an Excel
spreadsheet.
Feedback from testers has been positive and is helping the scientists improve the model. Further research has shown that the growth
rate of the pollen tube varies depending on the fruit variety and
weather conditions. For example, data from Washington orchards
shows that the time between pollination and fertilization can exceed
100 hours for Cripps Pink but that Fuji can be fertilized in just over
60 hours.
an apple flower
An apple flower
Pollen source
"It's a way
to add
precision
to the way
we're
using
bloom
thinners."
Questions have arisen about how different pollen sources might
affect the accuracy of the model. Manchurian is the most common
pollinizer in Washington, followed by Snowdrift.
—Gregory Peck
The data that the model was based on came mainly from the
maternal side of the pollination question—the fruit variety, Peck said. Now, the scientists
want to explore the impact of the paternal side (the pollen). Pollen used in developing
the model came from Snowdrift. He and his colleagues plan to study other pollinizers to
better understand the impact that the pollen source has on pollen tube growth rate. In
preliminary studies, when they looked at the growth rate of pollen tubes from eight different crab apples used to pollinate Gala flowers, they found a fourfold difference in
growth rate. A day after pollination, pollen tubes of the Thunderchild crab apple were
the longest; Snowdrift was intermediate; and Manchurian had among the shortest
pollen tubes.
In future research, the scientists plan to screen an array of crab apples and fruit varieties at different temperatures to assess the pollinizers' performance in the varied
weather conditions typically seen during bloom. They also hope to find out how
important the pollen source is in determining final fruit size and quality.
•
GOOD FRUIT GROWER March 15, 2013
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